Electrophotography



June 3, 1952 L. E. WALKUP ELECTROPHOTOGRAPHY Filed March 9, 1949 INVENTOR am's E 24%)? ATTORNEY Patented June 3, 1952 uni-Tao STATES PATENT D F F'I CE ELEC'IROPHOTOGRAPHY Lewis E. 'Wa'lkup, Columbus, 'Ohio, assignor, by mesne assignments, to The Haloid' Company, Rochester, N.Y., a'corporation 'of'New York Application March :9, 1949; Serial him-"80,551

.1 "Thisinvention relates to .electrophotography andhaslforits objectlto improve such processes.

Another purposeof. the invention is to improve therrendition of subject. matter having variations in tonal value throughout all or part of the area beingreproduced.

'Afurther. object is to improve the development of electrophotographic plates.

Other objects will be apparent from the following detailed description and the appended claims.

According to one aspect of the invention, the electrophotographic.plate, consisting .ofa layenof photoconductive insulating materialon a conductive backing, is exposedinsequence or .simultaneously to -a continuous tone subject-and a screen pattern. of uniformly distributed, finely divided light anddark areas.

In. the. drawings:

Fig. ..1. is an elevation, partly in section, illustratingthe exposure of acharged electrophotographic'plate to a continuous tone subject;

liig. 2 illustrates another. step comprising exposure of the charged plateto-ascreen-pattern;

Figs. 3, 4, 5, 6 and 7 are enlarged views of-small areas .of.five.-suitable screens, and

.Fig. '8 illustrates. amethod of-simultaneous exposure vto.a.continuous tone subjectanda screen pattern.

Whilea preferred. embodiment ofthe invention is described herein, it .is contemplated -that considerable variation. may be made in themethod of procedure and the construction of parts without departing from the spirit of the invention. In the following description, and in the claims, parts will 'be identified by specific .names, .for convenience," but they are intended'to be as=generic in'their application to similar ,parts'as the art will permit.

In the process of electrophotography as disclosed in Carlson Patent No. 2,297,691, a plate comprising a coating of photoconductive insulating material "on a conductive backing is given a uniform electric charge over'its surface and is then exposed "to the subject .matter to be reproducedby camerayprojection or contact exposure. This exposure discharges the plate areas in ac-.- cordance With'the light'intensity which reaches them, thereby creating an electrostatic latent image on or in the plate coating. "Development isefiected'with a'fin'ely divided material, such as "an electroscopic powder, which is brought into contact withthe coating and ishel'd thereon electrostati'cally in a pattern corresponding. in general,totheelectrestatic"latentimage. .iFinaI- 2 ly, the powder irnage is transferred to a suitable base, suchzasv paper, and. fixed thereon by fusion or .use o'f- -a solvent.

One development method comprises dusting the-platewith a finely divided, dyedresinpowder and removing the excess powder withan air blast. Another methodcomprisescascading over the-plate a developer which consists of a-iinely divided, pigmented resin powder clinging to a coarser granular carrier asdescribed ,in application for U.- S- Patent Ser. --No. 24,674, filed -May 1,.1948, by Lewis E. Wal-kup and Edward N. -Wise,- whereby. some of theresin powder is transferred to theplate in ---the-areas carrying the charge image.

- Excellent resultsl have been obtained with this process for line work-such as black letters or lines on a white-background, and forhalf-tones. On continuous tonerendition, such as is necessary in portraiture, pictorial photography, the copyingof'X-ray radiographs'and the like, the results have tended to be somewhat cont'rasty, however. A smoother gradation of tone -=would bedesirable, and -also a more'uniform rendition of large black areas.

'The present invention-is intended to afforda method and apparatus for improving continuous tone rendition and the rendering-oflargeblack areas.

Referring to the drawings, in which certain embodimentsof the invention areillustrated, Fig. lshows one method of recording a continuous tone electrostatic latent image on an electrophotographic plate. In-this-method the subject I, such as a person or thing to be reproduced, is posed before a conventional camera z provided with the usual lens andshuttermechanism 3 and is illuminated by a-light'source 4. The plate 5 comprising a conductive backing layer 6- coated with alayer of photoconductive insulating-material '1, 'suchasianthra'cene, sulfur or selenium,is held inplateholderfl inposition for exposure in the back of the camera.

Prior to insertion inthe camera the surface of layer 1 has applied to it a. uniformly distributed electrostatic charge by frictionally rubbing it with a cloth or brush .or by passing it under a corona discharge-wire or rowof needleaas shown and described, 'fonexample, in application for U. S. Letters Patent Ser.No. 55,526, fiIed October 20, 1948, byJohn-J. Rheinfrankjnow abandoned.

Exposureis made as shown-in Fig. 1 by conventional methods such as by flooding the subject -l with light and opening shutter 3 for a period .suflicient to substantially discharge the lamps l and having a glass top panel H which is covered with a thin transparent sheet l2 of cellophane, cellulose acetate or the-like which carries a screen pattern 13 printed or otherwise applied to its under surface. turned on momentarily to expose layer 1 to the image or shadow of the screen pattern which consists of alternate black and transparent areas.

This results in a corresponding electrostatic pattern on the layer I by substantially discharging the small areas or spots of the plate where light strikes it through the transparent areas, or at least reducing the charge on such areas to a fraction of its original value.

If exposure has previously been made to a con tinuous tone image as shown in Fig. 1, the plate is now ready for .powder development. If the screen exposure is made first, the plate is ready for the continuous tone exposure of Fig. l.

A variety of screen patterns may be used for screen l2, several examples of small areas of which are shown, greatly enlarged, in Figs. 3, 4, 5, 6 and 7. Thus in Fig. 3, the transparent areas l4 comprise rows of small squares entirely surrounded by black or opaque areas 15. In Fig. 4, a checkerboard pattern of transparent and opaque areas l6 and I1 is used. In Fig. 5, the transparent area I8 is continuous and is covered with circular black or opaque dots l9. In Fig. 6, the screen comprises a black or opaque background 21 covered with a random distribution of transparent dots of various sizes. Fig. 7 shows a further variation in which the screen comprises alternate rulings of transparent lines 22 and opaque lines 23.

Any one of several screen sizes may be used, the finer sizes generally giving the more natural or higher quality results in continuous tone rendition. Thus, While a coarse screen having or dots or lines to the linear inch will be useful for some purposes, such as in the direct production of half-tone images, finer screens, such as those having 100, 200, 300, 400 and even more dots or lines to the inch will-give more nearly a continuous tone appearance to the finished print. With the finer screens the screen pattern may be barely perceptible as such in the finished print and the copy will have the appearance of a continuous tone photograph. The contrasty appearance obtained Without the use of the screen exposure step is eliminated or greatly reduced and large black areas are rendered with apparently uniform density throughout.

While it is not desired to be limited to any particular theory of operation, it is known from other experiments that the developing powder, upon deposition, tends to form a heavier deposit along the edges of black areas than in the center of the black areas. This is thought to be possibly due to the configuration of the electrostatic field above the plate, tending to form a concentration of electrostatic lines of force at the edges of charged areas. With the screen pattern super- The lamps are 4 imposed, the charged areas are broken up into smaller charged areas thereby altering the configuration of the force lines to avoid concentration at the edges of large areas and cause a large number of lesser concentrated force areas corresponding to the screen pattern.

Instead of exposing the plate to the screen pattern in the exposure box, as shown in Fig. 2,

it is possible to make the screen exposure in the camera of Fig. l by placing a sheet having a black-and-white screen pattern in front of the "camera and exposing the plate thereto either before or after the continuous tone exposure.

Such a black-and-white screen would be similar to any of the screens of Figs. 3 to '7 except that the transparent areas would be white, and the same effect can be had :by placing any of the screens of Figs. 3 to 7 against a white background while exposing.

It is also possible to secure the same result by placing a transparent screen such as shown in Figs. 3 to 7 carrying an opaque pattern in the camera in front of and adjacent to or in contact with the plate and expose to a white field or white opaque screen located at a distance in front of the lens, either before or after the continuous tone exposure, and Where reference is made to white areas or white background, any color that will reflect light and affect the electrostatic image on the plate may be used in place of white.

It is also possible, in making prints of Com tinuous tone photographs, for example, to carry out both exposures by contact without the use of a camera. For example, the original to be copied is substituted for screen I2 on the copying box 9 for the continuous tone exposure step preceding or following the screen exposure.

While the invention has been illustrated in Fig. 1 in the rendition of three dimensional subject matter it is obvious that the same method can be used for copying two dimensional material such as paintings, photographs, X-ray shadowgraphs and grained wood surfaces placed in front of the camera.

Fig. 8 illustrates a further method of exposure in which the plate 5 is exposed, simultaneously or in sequence, to a projected continuous tone image from film 24 projected by enlarger 25 and a screen pattern from film or slide 26 projected by enlarger 21. Upon development the results are substantially the same as with the other methods.

Although the invention has been described as particularly applicable for the rendition of continuous tone subject matter, it is also contemplated that it may be used to eiiect improvement in the copying of black and white material, particularly where the subject contains large black areas. In this case the copies present a more uniform density of rendition over the dark areas.

While the present invention, as to its objects and advantages, has been described herein as carried out in a specific embodiment thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

I claim:

1. In an, electrophotographic process wherein the surface of a layer of photoconductive insulating material is provided with an electric charge, exposed to a light image of the subject to be reproduced andv then developed by depositing a finely divided electrostatically attractable mate-- rial under the influence of the charge pattern rmaining on said layer, the method of improving large area reproduction and continuous-tone rendition comprising additionally exposing the layer to a finely-divided uniformly-distributed pattern of light and shadow prior to developing.

2. In an electrophotographic process wherein the surface of a layer of photoconductive insulating material is providedwith an electric charge, exposed to a light image of the subject to be reproduced and then developed by depositing a finely divided electrostatically attractable material under the influence of the charge pattern remaining on said layer, the method of improving large area reproduction and continuous-tone rendition comprising additionally exposing the layer to a finely-divided uniformly-distributed pattern of light and shadow after exposure to the light image and prior to developing.

3. In an electrophotographic process wherein the surface of a layer of photoconductive insulating material is provided with an electric charge, exposed to a light image of the subject to REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 506,109 Gerland Oct. 3, 1893 691,535 Ehlermann Jan. 21, 1902 1,394,797 Smith Oct. 25, 1921 2,221,776 Carlson Nov, 19, 1940 2,297,691 Carlson Oct. 6, 1942 OTHER REFERENCES Clerc, Ilford Manual of Process Work, Percy Lund, Humphries & 00., London, fourth edition 1946, page 307 cited.

Clerc, Photography-Theory and Practice, page 559, Pitman Publishing Corp. N. Y. 2d Ed. 1937. 

